Ergosterol is a type of sterols, which is contained in microorganisms such as fungi. It is a substance useful as a provitamin D2. It has been known that when such an ergosterol is crystallized in a water-insoluble organic solvent, granular aggregates exhibiting good solid-liquid separation character can be obtained (Japanese Patent Laid-Open No. 2002-80492). In this case, however, the actual yield of crystals is significantly lower than that estimated from the solubility of ergosterol in the above organic solvent. Thus, this method is problematic in that the remainder of ergosterol are gradually precipitated as fine crystalline flocculates from the filtrate after solid-liquid separation. It has also been known that ergosterol forms hydrate crystals (S. E. Hull et al., Acta Cryst. B32, 2370-2373 (1976)). However, the association of this fact with the aforementioned problem has not been clarified.